The present invention relates to a method for controlling a hybrid drivetrain which has a drive engine and a dual-clutch gearbox which, to establish two power transmission paths, has a first and a second friction clutch and a first and a second component gearbox, wherein an electric machine is connected or can be connected to the second power transmission path downstream of the second friction clutch in the power flow direction, wherein purely electric driving operation can be realized by means of the second component gearbox assigned to the second friction clutch, and wherein, in the event of a gear change in the second component gearbox during purely electric driving operation, a fill-in torque is provided.
Dual-clutch gearboxes of the described type are generally known. Through the provision of two power transmission paths, it is possible to perform gear changes without an interruption in traction force. Here, the drive torque provided by the drive engine is transferred from one power transmission path to the other without an interruption in traction force by means of overlapping actuation. Said type of control of a dual-clutch gearbox is known in particular in the case of drivetrains whose drive engine is an internal combustion engine.
It is also known for drivetrains of motor vehicles to be hybridised. This generally includes the provision of at least one electric machine, which can generally operate as an electric motor or as an electrical generator. In the operating mode as an electric machine, additional drive torque can be provided. In the operating mode as an electrical generator, the electric machine can operate in a recuperative manner in order to obtain, for example during a braking process, electrical energy for charging an energy store.
In general, in drivetrains of said type, it is known for an electric machine to be arranged between the drive engine and the dual-clutch gearbox, that is to say upstream of the friction clutches of the dual-clutch gearbox in the power flow direction.
In the case of the hybrid drivetrain mentioned in the introduction, however, the electric machine is, within the dual-clutch gearbox, connected at least intermittently to an input of one of the component gearboxes, that is to say downstream of the associated friction clutch (in the present case of the second friction clutch) in the power flow direction. In this way, a summing point of the drive engine and electric machine is situated not upstream of the input of the dual-clutch gearbox but rather in the gearbox. In this way, the shift sequences of a conventional dual-clutch gearbox can be enhanced.
With a drivetrain of the described type, conventional operation is possible in which only the drive engine is active but the electric machine is not connected to the drivetrain or is concomitantly cranked. A hybrid operating mode is also possible in which the drive power is provided both by the drive engine and also by the electric machine. Finally, purely electric driving is also possible in which the internal combustion engine is shut down (that is to say, in general, is at a standstill). Here, the friction clutches of the dual-clutch gearbox are generally open in order that the drive engine (generally an internal combustion engine) need not be concomitantly cranked.
In the purely electric driving mode, the conventional shift sequences of a dual-clutch gearbox cannot be used because the electric machine is situated on the secondary side of the associated friction clutch.
Document DE 10 2010 044 618 A1 discloses methods for controlling a hybrid drivetrain during purely electric driving operation. Here, an electric machine is connected downstream of a clutch of a dual-clutch gearbox in the power flow direction, such that electric driving operation can be realized by means of the gear stages of the associated component gearbox. Here, in the event of gear changes, traction force assistance can be achieved in the following way.
In a first variant, the internal combustion engine is cranked, without being fired, by virtue of one or the other clutch being closed. During the gear change (upshift), via the other clutch, a traction force assistance torque is provided which utilises the inertial energy of the then coasting-down internal combustion engine. Here, a gear stage change may also take place in the component gearbox assigned to the other clutch in order to crank the internal combustion engine with a suitable gear stage and provide the traction force assistance torque with a different gear stage.
In a second variant, an overrun force assistance torque can be provided during downshifts by means of similar steps, wherein here, the internal combustion engine is not cranked up to speed but rather is used, at a standstill, as a drag load. Here, the overrun force assistance torque should be regulated such that the internal combustion engine is not set in rotation, that is to say a torque exerted on the internal combustion engine remains below a so-called breakaway torque.